It is known that it is desirable to produce elastomeric polymers capable of exhibiting reduced hysteresis when properly compounded with other ingredients such as reinforcing agents and then vulcanized. Such elastomers, when fabricated into components for constructing articles such as tires, vibration isolators, power belts, and the like, will manifest properties of increased rebound, decreased rolling resistance and less heat-build up when subjected to mechanical stress during normal use.
The hysteresis of an elastomer refers to the difference between the energy applied to deform an article made from the elastomer and the energy released as the elastomer returns to its initial, undeformed state. In pneumatic tires, lowered hysteresis properties are associated with reduced rolling resistance and heat build-up during operation of the tire. These properties, in turn, result in lowered fuel consumption of vehicles using such tires and prolonged tire life. In such contexts, the property of lowered hysteresis of compounded, vulcanizable elastomer compositions is particularly significant. Examples of such compounded elastomer systems are known to the art and are comprised of at least one elastomer (that is, a natural or synthetic polymer exhibiting elastomer properties, such as a rubber), a reinforcing filler agent (such as finely divided carbon black, thermal black, or mineral fillers such as clay and the like) and a vulcanizing system such as sulfur-containing vulcanizing (that is, curing) system.
Various synthetic strategies have been developed to provide elastomers with molecular structures exhibiting reduced hysteresis energy losses. One technique is to produce elastomers of very high molecular weight. In such high molecular weight systems, the number of free, uncrosslinked molecular chain-ends per given weight in the vulcanizates made from them are reduced. Since the presence of free, unbound chain ends are believed to be a significant factor in hysteretic energy loss because they cannot participate in elastic recovery processes, their reduction leads to a desirable reduction in hysteretic energy loss.
Another technique is to prepare elastomer molecules with end groups capable of interacting with the reinforcing fillers such as carbon black present in compounded elastomer compositions. Again, such interaction reduces the number of free end groups believed to contribute to hysteretic losses. Such interactive end groups include those derived from various metal reagents as well as those derived from polar organic reagents such amines, amides, esters, imines, imides, ketones and various combinations of such groups. For example, commonly assigned U.S. Pat. Application Ser. No. 636,961 filed Jan. 2, 1991, describes elastomers with tin containing end-groups derived by initiating polymerization under anionic conditions with tin-lithium compound such as trialkyl tin (IV) lithium that is, (alkyl).sub.3 SnLi groups. This application does not disclose or suggest reaction of polymer with a 1,3,2-dioxastannolane which, under the anionic conditions used in this invention, occurs by opening of the tin-oxygen ring to form a carbon-tin bond. Thus the reactions occuring in the present invention are inherently different than those described in the above-noted reference.
Another approach to elastomers with reduced hysteresis properties of elastomer compounds involves coupling of lithium-terminated elastomer intermediates with tin halides. In such coupling reactions two or more elastomer molecules are joined or coupled through a common tin atom. U.S. Pat. Nos. 4,383,085 and 4,515,922 describe coupling of lithium-terminated polymers with tin or silicone halides. An article in Rubber Chemistry and Technology 63, pp8-22, by F. Tsutsumi et al (1989) describes coupling of solution prepared styrene/butadiene rubbers with tin compounds tri- and tetra-chlorotin compounds.
The present invention is directed to 1,3,2-dioxastannolane-modified elastomers which, when compounded and vulcanized by known rubber processing techniques, provide vulcanized elastomers which exhibit desirable low hysteresis characteristics. These dioxastannolane-modified elastomers can be made by reacting lithium - terminated elastomer intermediates with more than about 0.4 equivalents of at least one 1,3,2-dioxastannolane. The stannolane elastomers thereby produced contain oxygen-tin-carbon moieties which either cap or couple the intermediate elastomer molecules. Mixtures of 1,3,2-dioxa stannolane modified elastomers with unmodified elastomers derived from the lithium-terminated elastomer are also useful.